1. Field of the Invention
The present invention relates in general to the field of integrated circuits and in particular to an assembly on which multiple integrated circuits may be mounted.
2. Description of the Related Art
As the demand for manufacturing electrical assemblies with greater densities has increased, various techniques have been developed to utilize the available area on the printed circuit boards used in such assemblies. One such technique known as chip-on-board (COB) eliminates the need for chip carriers or packages. The COB method involves mounting integrated circuit chips directly on the printed circuit board (PCB). During the mounting process the chip is first flipped over so that the bonding pad side faces downward. The chip is then mounted to the PCB board using a control-collapse-chip-connection (also known as C4). In the C4 process, the chip is connected to the PCB by interposing a plurality of interconnect bumps between the bond pads on the chip and a series of corresponding bump pads on the upper surface of the PCB. There is typically another series of bump pads and bumps on the underside of the PCB to connect the PCB to another component. The PCB includes plated through holes that connect the bottom bump pads to the top bump pads.
There are several disadvantages associated with the conventional COB method. First, the use of bumps elevates the chip above the PCB, leaving a gap as wide as the diameter of the bumps. If there are mismatches in the coefficients of thermal expansion of the chip and the PCB, thermal cycling may lead to stress failure of the bumps. In an attempt to alleviate the thermal stress problem, designers have dispensed underfil materials into the gap between the chip and the PCB. However, the underfil material must normally be applied after the chip is in place. This is a time consuming process that requires specialized equipment. In addition to subjecting the chip connection to potentially damaging thermal stresses, the conventional COB process requires time consuming and expensive processing steps and a dual-camera alignment system to form the bumps and bump pads.